Copper strike bath and method for coating electrolessly plated articles

ABSTRACT

A copper strike electroplating bath comprising tetrapotassium pyrophosphate, potassium cyanide and cuprous cyanide. The strike is especially useful for plating on electroless metal deposits such as nickel.

United States Patent [191 Lerner 5] Dec. 23, 1975 COPPER STRIKE BATH AND METHOD FOR COATING ELECTROLESSLY PLATED [56] References Cited ARTICLES FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Lewis Brian Lerner, Linglestown, 94,652 6/1960 Netherlands 204/52 Y Pa. Primary Examiner-G. L. Kaplan [731 Assignee' AMP Incorporated Hamsburg Attorney, Agent, or Firm-Russell J. Egan, Esq. [22] Filed: June 28, 1974 I 21 Appl. No.: 484,023 [57] ABSTRACT A copper strike electroplating bath comprising tetrapotassium pyrophosphate, potassium cyanide and cufi 5 8; 204/38 Y prous cyanide. The strike is especially useful for plati i z B ing on electroless metal deposits such as nickel. 106/1, 6 Claims, No Drawings 1 COPPER STRIKE BATH AND METHOD FOR COATING ELECTROLESSLY PLATED ARTICLES BACKGROUND OF THE INVENTION In the art of electrolytically depositing various metals, it has been very important that good adhesion is obtained between the substrate and the electrolytically deposited metal. A gOOd adhesion is essential when passive electrolyte deposits are being produced by a technique known as electroless nickel or copper deposltlon.

additional plating operations, such as the electrolytic plating operations previously mentioned above.

As can well be appreciated, not every electrolyte bath is a good bath for obtaining a strike Thus, the

particular combination of the elements in the invention herein have been found to be suitable for the purposeof achieving the strike" effect especially between the copper strike and the passive, electroless nickel deposit without being necessarilyuseful for some other pur pose. a

The term strike is defined as a deposit sufficient to ,cover the underlying deposit and improve its conductivity by means of a thin (I to microinch) layer ofthe strike metal. 7

According to electroless nickel deposition, thin film,

passive electroless metal deposits are obtainedby appropriately causing to react on the surface a catalyst, which is on the surface of thesubstrate, a very light deposit of metal, such as, by immersion in a bath containing the metal in solution. The film, which is formed on the substrate, is then exposed to a strike which cooperates especially well with the thin film deposit so that the subsequent layers of metals sought to be deposited will adhere between the electroless deposit and the electrolytic deposit.

Furthermore, it is well known that in order to pro duce tightly adhering deposits on an'organic substrate, such as a polyimide film, polyimide-amide film, etc., a catalyst must furnish a sufficiently tight bondwith the substrate so that the metal from the electroless bath would deposit on the surface sufficiently tightly,with a good bond between the substrate and the metal in the electroless bath. As it can well be appreciated, the adhesion between the catalyst and the metal deposited from the electroless and electrolyte bath is fairly critical and 'numerous proposals have been made to solve the adhesion problem.

In addition, in order to improve the adhesion between the electroless metal and the electrolytically deposited metal without affecting the catalyst to electroless metal bond, various baths known as strike baths have been used to achieve a deposit for subsequent immersion of the substrate in another bath for electrolytical deposition of the metal sought to be deposited. Often the metal which is being deposited on the strike is a noble metal, i.e., gold or silver.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, it has now been discovered that a copper cyanide bath ofa particular combination of elements is eminently suitable for use with thin film, passive electroless metal deposits such as nickel to achieve a deposit ofa strike of good quality. The novel bath displays an ability to strike" deposit copper on the surface and cover the electroless metal e.g. nickel deposit immediately with copper.

As it covers the electroless nickel metal deposit, the strike also provides very good adhesion. In addition, the strike deposit builds up to nominal thickness such as from 2 to l() microinches of copper thereby reducing the electrical resistance in the substrate for subsequent In accordance with the invention, the following embodiment is illustrative of the same and is illustrated by the following solution made up as a 1 liter solutio useful as a strike electrolyte.

TABLEI ELECTROLYTE SOLUTION Solution Make Up gr/liter Range gr/liter a. Tetrapotassium I20 I50 Pyrophosphate K,P O .3I-I O* b. Potassium Cyanide KCN** I23 4 17 c. Cuprous Cyanide CuCN 8.2

Ratio of h. to c. is 2:l to 4:1 by weight Proportionate to c. in the indicated ratio With reference to the above disclosed bath, it has been used for the deposition of a strike of a thickness from 2 to 10 microinches on a Kapton H film having a nickel or cobalt deposit thereon. These deposits have been prepared as follows.

A Kapton polyimide polymer film of a thickness of 3 mils is immersed from 1 to 3 minutes in an aqueous solution of equalparts of 25% sodium hydroxide, by weight, 85% hydrazine hydrate, by weight, and' trietha'nol amine 10% solution by weight. Thereafter the film is sprayed or immersion rinsed with cold water, followed by immersion intoa 1%, by weight, palladium chloride aqueous hydrochloric acid solution, wherein palladium chloride isof a concentration of 0.1 to 1 gram per liter and hydrochloric acid is 5 to 10 milliliters per liter. In addition, a stannous chloride (10 grams per liter) and hydrochloric acid (40 milliliters per liter) solution may be used in conjunction with the palladium chloride solution as a catalyst, immersed in cold water, and dried at 1009C for 2 minutes. The surface accepts nickel or cobalt from one of the following:

Nickel Chloride 8t) gm/l Sodium Citrate I00 gm/l Ammonium Chloride 50 gm/l Sodium Hypophosphite l0 gm/l Bath Temp: F Nickel Chloride Hexahydrate 20 gm/l Ethylene Diamine (98%) 45 gm/l Sodium Hydroxide 4O gm/l Sodium Borohydride 0.67 gm/l Bath Tcmp.: l80F Cobalt Chloride Hexahydrate 30 gm/l Sodium Citrate Pentahydrate 35 gm/l Ammonium Chloride 50 gm/l Sodium Hypophosphite. Monohydrate 20 gm/l Bath Temp.: l80F Cobalt Sulphate I-Icptahydrate 24 gm/l Ammonium Sulphate 40 gm/l Sodium Hypophosphite 20 gm/l Sodium Citrate 80 ml] Sodium Lauryl Sulphate 0.1 gm/l Bath Temp: l80F The above treated film when immersed in the abovenickel or cobalt solution deposited nickel or cobalt in thicknesses which were sufficient for additive, semiadditive, or subtractive plating. These deposits were then covered with the copper strike.

Besides the Kapton (polyimide) film, poly imidea mides and poly (parabanic acid) polymers may be used. The last two are of the polyimide polymer type family. Poly (parabanic acid) polymers are described in U.S. Pat. Nos. 3,547,897; 3,591,562 and 3,661,859, the disclosures of which are incorporated by reference herein. The polyimides and polyimide-amides are described in Lee et al., New Linear Polymer, McGraw- Hill, N.Y., N.Y. (1967) pp. 171 et seq.

To obtain the strike deposit on the nickel or cobalt deposits, the above bath was used under the following operating conditions.

As part of operating said bath, continuous filtration is necessary to remove any precipitates and impurities and to prevent auto-decomposition of the bath.

As obtained from the above bath, the strike deposit was tested by a Scotch tape adhesion test and the total electroless and strike deposit by adhesion test known in the industry as a 90 peel test and it was found that the adhesion was very satisfactory both for the strike-electroless adhesion by the Scotch tape test and the 90 peel test for the total deposit. The last test also indicates that the strike bath does not affect the adhesion of the electroless metal to the substrate.

According to the above-identified method, the following substrates of which an electroless and strike coating are deposited are: polyimides; polyimidesamides; poly (parabanic acid); polyester film (Mylar film); polyethylene film, etc.

The above strike covered deposits which are electroplated are used such as in printed circuit boards, circuits for integrated circuit chip attachment used in hand held calculators, etc.

What is claimed is:

l. A method for depositing a strike electrolytically on a substrate the steps comprising: depositing on a substrate an electroless layer of metal by catalyzed deposition or by immersion deposition; exposing said electroless layer on said substrate between an anode and a cathode in a bath which is at a pH of about 6.8 to 7.2 and of a composition which comprises: tetrapotassium pyrophosphate K P O .2H O from about to 150 grams per liter; potassium cyanide fromabout 4 to 17 grams per liter; and cuprous cyanide wherein the ratio of potassium cyanide to cuprous cyanide is from 2:1 to 4:1 by weight; impressing a current between said anode and said cathode to electrolytically cause the deposition of copper from said solution onto said substrate having said electroless or immersion deposited layer; and building upon said electroless or immersion deposit a tightly adhering layer of high purity copper in a thickness of 2 to 10 micro inches.

2. An aqueous bath for depositing electrolytically a copper strike on an immersed body having a metal deposit thereon, the copper strike bath being at about a pH of 6.8 to 7.2 and consisting essentially of the following components: tetrapotassium pyrophosphate K P O .3H O from 90 to l50 grams per liter; (a) potassium cyanide from 4 to 17 grams per liter; and (b) cuprous cyanide, wherein the ratio of a to b is 2:] to 4:1, by weight.

3. A method for depositing a copper strike electrolytically on a substrate having an electroless or immersion deposited layer as a conductive layer the steps of the method comprising inserting the substrate as a cathode in a bath which is at about a pH of 6.8 to 7.2 and consisting essentially of tetrapotassium pyrophosphate K P O .3H O; (a) potassium cyanide; and (b) cuprous cyanide, wherein the ratio of a to b is 2:1 to 4:1, by weight, impressing a current between an anode and said cathode to electrolyze said bath and cause a deposition of copper from said solution unto said substrate and building upon said substrate a tightly adhering layer of high purity copper in a thickness of 2 to 10 microinches.

4. The process as defined in claim 3 wherein the temperature of the bath is from to l40F and the pH of the bath at 25C is from 6.8 to 7.2.

5. The process as defined in claim 3 wherein the bath is vigorously agitated and the current is impressed between the anode and cathode between at 10 to 30 ASP of said cathode.

6. The process as defined in claim 3 wherein the specific gravity of the bath at 25C, is 14 to 22Be. 

1. A method for depositing a strike electrolytically on a substrate the steps comprising: depositing on a substrate an electroless layer of metal by catalyzed deposition or by immersion deposition; exposing said electroless layer on said substrate between an anode and a cathode in a bath which is at a pH of about 6.8 to 7.2 and of a composition which comprises: tetrapotassium pyrophosphate K4P2O7.2H2O from about 90 to 150 grams per liter; potassium cyanide from about 4 to 17 grams per liter; and cuprous cyanide wherein the ratio of potassium cyanide to cuprous cyanide is from 2:1 to 4:1 by weight; impressing a current between said anode and said cathode to electrolytically cause the deposition of copper from said solution onto said substrate having said electroless or immersion deposited layer; and building upon said electroless or immersion deposit a tightly adhering layer of high purity copper in a thickness of 2 to 10 micro inches.
 2. AN AQUEOUS BATH FOR DEPOSITING ELECTROLYTICALLY A COPPER STRIKE ON AN IMMERSED BODY HAVING A METAL DEPOSIT THEREON, THE COPPER STRIKE BATH BEING AT ABOUT A PH OF 6.8 TO 7.2 AND CONSISTING ESSENTIALLY OF THE FOLLOWING COMPONENTS: TETRAPOTASSIUM PYROPHOSPHATE K4P2O7.3H2O FROM 90 TO 150 GRAMS PER LITER; (A) POTASSIUM CYANIDE FROM 4 TO 17 GRAMS PER LITER; AND (B) CUPROUS CYANIDE, WHEREIN THE RATIO OF A TO B IS 2:1 TO 4:1, BY WEIGHT.
 3. A method for depositing a copper strike electrolytically on a substrate having an electroless or immersion deposited layer as a conductive layer the steps of the method comprising inserting the substrate as a cathode in a bath which is at about a pH of 6.8 to 7.2 and consisting essentially of tetrapotassium pyrophosphate K4P2O7.3H2O; (a) potassium cyanide; and (b) cuprous cyanide, wherein the ratio of a to b is 2:1 to 4:1, by weight, impressing a current between an anode and said cathode to electrolyze said bath and cause a deposition of copper from said solution unto said substrate and buildIng upon said substrate a tightly adhering layer of high purity copper in a thickness of 2 to 10 microinches.
 4. THE PROCESS AS DEFINED IN CLAIM 3 WHEREIN THE TEMPERATURE OF THE BATH IS FROM 120* TO 140*F AND THE PH OF THE BATH AT 25*C IS FROM 6.8 TO 7.2.
 5. The process as defined in claim 3 wherein the bath is vigorously agitated and the current is impressed between the anode and cathode between at 10 to 30 ASF of said cathode.
 6. The process as defined in claim 3 wherein the specific gravity of the bath at 25*C, is 14* to 22*Be. 